The present invention relates to a printing mechanism for a dot matrix printer, and more particularly to a printer comprising a platen having a plurality of protrusions helically provided on the periphery of the platen, and a plurality of hammers arranged in the axial direction of the platen, each of the hammers has a hammer head which is provided to cross protrusions, which is called a cross point type dot matrix printer.
Generally, the dot matrix impact printer is compact and economical and produces a clear print on a recording paper. The cross point type dot matrix impact printer is a more compact and economical printer. Generally, the cross point type dot matrix impact printer has a platen having a helical projection, a plurality of hammers arranged in the axial direction of the platen to cross the helical projection. The hammer strikes the helical projection interposing the recording paper, so that dot matrix printing is performed. The form of the printed dot is a small square made by a crossing area of the helical projection of the platen and the hammer head.
In such a dot matrix printer, the distance between the surface of the helical projection and the surface of the hammer head at the circumferentially opposite end portion of the crossing area is larger than the distance at a central portion of the crossing area, since the surface of the helical projection is circularly curved. Therefore, the printed dot is unclear at circumferentially opposite end portions. As a result, letters and figures composed by such dots are illegible.